Electrical resistance welding of coated sheet metals

ABSTRACT

The invention provides a method for the electrical resistance welding of coated sheet metals, in which welding current is fed to at least one of the coated sheets indirectly through the metal layer thereof and contact is established between blank areas thereof to be welded to each other, wherein annular or elongated knife edges are provided at the welding area of the sheet metal to be welded on or on intermediate welding pieces, and cavities are produced by embossing and/or by milling, for preserving an outer coating undamaged by the heat of welding.

United States Patent 72] Inventor Alfred Otto Becker 56 Robert RockStrasae, D-6600 Saarbrucken, Germany [2| Appl. No. 869,668

[22] Filed Oct. 27, 1969 [45l Patented Dec. 21, 197] [32 Priorities Oct.26, 1968 [33] Germany Nov. 7, 1968, Germany, No. P 18 07 382.0; May 31,1969, Germany, No. P 19 27 915.3; Sept. 3, 1969, Germany, No. P 19 44614.]; Sept. 20, 1969, Germany, No. P 19 47 771.5

[54] ELECTRICAL RESISTANCE WELDING 0F COATED SHEET METALS 25 Claims, 29Drawing Figs.

[52] US. Cl 219/93, 219/91 [51] Int. Cl 323k 9/28, B231; 1 ill 0 [50Fieldol Search 219/91.92, 93

[56] References Cited UNITED STATES PATENTS 3,067,488 12/1962 Bennett etal 219/91 X 3,418,444 12/1968 Ruehlemann 219/91 X Primary Examiner-.1.V. Truhe Assistant ExaminerHugh D. .laeger Attorney- Erich M. H. RaddeABSTRACT: The invention provides a method for the electrical resistancewelding of coated sheet metals, in which welding current is fed to atleast one of the coated sheets indirectly through the metal layerthereof and contact is established between blank areas thereof to bewelded to each other, wherein annular or elongated knife edges areprovided at the welding area of the sheet metal to be welded on or onintermediate welding pieces, and cavities are produced by embossingand/or by milling, for preserving an outer coating undamaged by the heatof welding.

ELECTRICAL RESISTANCE WELDING OF COATED SHEET METALS I CROSS-REFERENCESSer. No. 668,414 Wefers and Becker, filed Sept. I8, 1967 Ser. No.704,684 Becker, filed Feb. 12, I968 Ser. No. 823,077 Becker, filed May8, I969 BACKGROUND TO THE INVENTION Sheets coated with plastics are verydesirable owing to the protection from rusting which the plasticscoating gives, and also due to the fact that the plastics coatingenables the sheets to be given any desired color or surface pattern.Sheets of this kind cannot however be connected with each other to forma composite sheet structure by the usual spot-welding or seamweldingtechniques, owing to the presence of the plastics coating. I havealready proposed to enable connection of such sheets, by removing thecoating from the sheets at the welding regions so that when the sheetsare pressed against each other, an electric contact is established.

With thick plastics coatings and also with thick sheets and other parts,the cavities which are produced by removal of the coating or byembossing or by the insertion of intennediate welding pieces, arebridged over. Consequently, with conventional welding machines,faultless welded connections can be established, even with sheets coatedwith plastics, lacquers or other materials, or sheets or metal partsmade of steel, chrome nickel steel, anodized aluminum or other metals.However, the external coatings of the welded sheets are destroyed at theweld regions, either by having been removed to enable the weldingaction, or by having melted under the electrodes.

I have already proposed for the welding current to be conductedindirectly through the sheet surface, for at least one of a number ofcoated sheets to be welded together. As a result of this, the externalcoating of at least one of the sheets remains undamaged since thewelding takes place internally to the opposite bare sheet surface. Whencapacitor pulse welding is used, pointed formations should be providedin the cavity, on the sheets or on intermediate welding pieces disposedbetween the sheets.

It is disadvantageous to weaken the welding regions on the sheet thecoating of which is to remain undamaged, and further difiiculties arecaused by thehigh local heating which the pointed formations result inat the weld regions.

It is an object of the invention to provide a method for the electricresistance welding of coated sheets enabling at least one of the sheetsto remain undamaged at its outer side while nevertheless allowing theeasy provision of formations facilitating the welding.

It is another object of the invention to provide a method for theelectric resistance welding of coated sheets, in which welding currentis fed to at least one of the sheets indirectly through the sheetsurface and electric contact is established between the sheets byring-shaped or line-shaped contact edges formed on one of the sheets oron an intermediate piece disposed between the sheets, by embossing ormilling.

With suitable formation of the contact edges thick and thin sheets orsheets or different metals can be joined by electricresistance weldingand by capacitor-discharge welding, the heating of the sheets being soslight that the outer coatings remain undamaged. This will be explainedin a series of examples.

BRIEF SUMMARY OF THE INVENTION serving as the decorative side, remainsundamaged. For the.

accurate production of the small ring-shaped edge, the milling tool maybe guided in a guide sleeve which is centered by engagement around theoutside of the embossed region.

,'In accordance with a second example, the milling tool may be simplyguided in the die so that immediately after the embossing and withouthaving to remount the embossed sheet, a cavity and a sharp ring-shapededge can be produced.

In a third example, a ring-shaped edge with a surrounding trough-shapedformation can firstly be milled into the sheet at the welding region,whereafter embossing is effected by means of a punch or by means of theelectrode which will subsequently be used for the application of weldingcurrent.

According to a fourth example, for seam welding of the sheet, astrip-shaped embossing is performed and then grooves are milled out soas to form line-shaped contact edges. The

milling cutters may be of disc-shape guided in the embossing die.

In accordance with a fifth example, for thick sheets the sheet is alsomilled out above the welding region.

In the sixth and seventh examples, two further forms of the weldingregions will be referred to, enabling the outer coatings of the upperand lower sheets to remain undamaged. For achieving these shapes of thewelding regions, external and internal milling cutters with suitableshapes are used.

In accordance with an eighth example, with sheets which are too thin toallow milling, ring-shaped or'line-shaped contact edges and associatedcavities are simply embossed.

In a ninth example, welding strips with ring-shaped edges and cavitiesare used. They may be firmly retained in a stepped bore or may have aprojecting head and be inserted in a continuous bore.

In a 10th example, the contact between the sheets may be further reducedby the use of welding balls. For inserting the welding balls, grooveswith triangular cross section are milled out at the welding regions orthe sheet to be welded on is provided with bores for receiving theballs.

According to an llth example, a welding point is milled on the sheet,and an intermediate welding piece is inserted, the lower side of whichhas a ring-shaped contact edge and a cavity.

In a 12th example, there are placed between the sheets small hollowintermediate welding pieces, which are provided upwardly with a pointand downwardly with a ring-shaped contact edge, or both upwardly anddownwardly with ringshaped contact edges.

According to a 13th example, with sheets coated only on one side, andwith the interposition of an insulating sheet having perforations, atthe welding regions, in each case a ringshaped intermediate weldingpiece of metal sheet, wire, or in the form of a plate with upwardly anddownwardly directed spikes, is inserted.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING The above objects ofthe invention, and the examples discussed, will be clear from thefollowing description which is given by way of example with reference tothe accompanying drawings, in which:

FIGS. 1, 2 and 3 are sectional views illustrating three stages in theformation of a welding region for sheets coated on both sides, providinga cavity and a ring-shaped contact edge in the upper sheet, by embossingand milling;

FIGS. 4 and 5 are sectional views illustrating the production of awelding region with contact edges of rectangular section,

and illustrating the welding operation;

FIGS. 6, 7 and 8 are sectional views of a third example, showing themilling of a ring-shaped contact edge, the embossing of the contactregion, and the disposition of the two sheets when ready to be welded;

FIG. 9 is a sectional view illustrating the production of a weldingregion for seam welding, with striplike embossing and milling ofrectilinear cavities and contact edges;

FIGS. and 11 are sectional views relating to a fifth example, in whichmilling is performed also on the upper side of the sheet, in the case ofthick sheets;

FIGS. 12 and 13 are sectional views relating to a sixth example, showingthe milling of. an, inwardly tapering ring-shaped contact edge;

FIGS. 14 and 15 relate to a seventh example, showing the milling of anoutwardly tapering ring-shaped contact edge;

FIGS. 16 and 17 relate to an eight example, showing two differentformations of cavities and contact edges, produced simply by embossing;

FIGS. 18 and 19 relate to a ninth example, showing milling through ofthe upper sheet, and the insertion of rivetlike members having cavitiesand annular contact edges;

FIGS. 20 and 21 relate to a 10th example, showing the interposition ofwelding balls between the sheets to be welded together;

FIGS. 22 and 23 relate to an llth example, showing the provision of aconical-welding point in one of the sheets, and an intermediate weldingpiece having a cavity and a ringshaped contact edge;

FIGS. 24, 25 and 26 relate to a l2th example and show three differentarrangements using hollow intermediate welding pieces having a pointedformation extending upwardly and a ring-shaped contact edge formationextending downwardly; and

FIG. 27, 28 and 29 illustrate the formation of a welding region forsheet coated on one side only, and showing the use of an insulatingsheet between the sheets to be welded, with the provision ofintermediate welding pieces having an annular shape.

DETAILED DESCRIPTION OF THE INVENTION In FIGS. 1 to 3, a sheet 1 isillustrated on an enlarged scale, the sheet having a coating la on itsupper side and a coating lb on its'lower side. At the region wherewelding is to take place, the coating is removed from the upper side ofthe sheet at the region 1c, and from the lower side of the sheet at theregion 10, so as to facilitate spot welding. The sheet is placed on adie 4 having a die opening 4a. By means of a punch 3 disposed above thesheet, the sheet is provided with a cylindrical embossed portion 1d andsubsequently a cavity la is formed on the lower side of the embossedportion by means of a rotating spherical surfaced end milling cutting7a. As shown in FIG. 2, the milling cutter is guided by a guide sleeve 6which is accurately centered by engaging over the margin of the embossedregion of the sheet. Stop means, not shown, limit the depth of action ofthe milling cutter, so that a sharp edge la" is formed. By relating theheight of the contact edge 1e" and the depth and angular disposition ofthe cavity to the welding parame ters such as welding time, voltage,current strength, resistance welding and possibly also the establishmentof arcs between the bare sheet surfaces to be welded, is enabled.

The prepared sheet 1 is placed above a sheet 2 to be welded on, havingthe coatings 2n and 2b and a region 2c in the coating 20, where thecoating has been removed, in such a manner that the embossed region ofthe sheet 1 lies coaxially to the stripped region 20 (see FIG. 3). Afterconnecting the sheet 2 to a welding installation, as diagrammaticallyindicated by an arrow in FIG. 3, the welding is performed by pressingdown the electrode 5. The sharp contact edge produces, depending on itsangular position, an accurately predeterminable electric resistance, sothat if desired, in addition to the resistance welding, electricarcs mayoccur in the cavity, causing the margin of the cavity to be melted downso that the sheets are welded to each other. The heat produced at thering-shaped welding region is rapidly conductedinto the sheet body andthere is only slight heating of the outer side of the sheet. Thedecorative lowermost coating layer 2b remains undamaged if the electricresistance of the contact edge 1c" and the welding parameters aresuitably selected.

For thick sheets, the arrangement according to the second embodimentshown in FIGS. 4 and 5, is suitable. A sheet 1 having the coatings la,lb has a stripped region lc' only at the lower side, and is providedwith a cylindrically embossed region. For producing a ring-shapedwelding region, the embossed region is cylindrically milled out at 1}".For doing this, the shank of an end milling cutter 7b is guided in theopening 4a of the die 4. For supplying a cooling medium, and forenabling removal of the material milled out passages 4b, 4c are providedin the die. The end-milling cutter, which is of smaller diameter thanits shank cuts a cylindrical recess leaving an edge If of rectangularcross section. The milling out and the embossing can be performed in avery short time. The amount by which the cutting head of the millingtool 712 is reduced in diameter determines the width of the materialconstituting the ring and the desired transition resistance is obtainedby selecting this width. For arc welding, the width of the ring and thedepth of the milled region should be so selected that electric arcs canoccur at the bottom of the milled region. The prepared sheet 1 is thenplaced above a sheet 2 to be welded, and which is coated on both sidesand which on its upper side has a region 20 free of coating, and isconnected by a cable 9 to a capacitor-welding installation C, the lowersheet 2 also being connected to the welding installation by a cable 10.The welding is performed under pressure exerted by a punch 8, which doesnot carry current. The electric resistance at the edge If" and otherfactors determine whether only resistance welding or simultaneously anarc welding takes place. The height of the contact edge 1f depends onthe depth of milling of the embossed region, and can be selectivelydetermined by preferably adjustable abutments which are e.g., arrangedon the milling cutter 7b or on the die 4, in order to obtain an optimumelectric resistance in combination with the width of the contact edge.The height of the step in the milling cutter may also be chosen so thata precise constant height of the contact edge If" is achieved.Accordingly, it is possible for the electric resistance of the contactedges to be accurately reproducable by means of milling, so that incombination with selection of the welding parameters in accordance withthe remaining relations, optimum welding results can be achieved. Theoutwardly lying coatings remain undamaged if the electric resistance andthe welding parameters are suitably selected.

In accordance with the third example, illustrated in FIGS. 6 to 8, thewelding region is milled first, so that the embossing is made easier, orso that the embossing can be performed with the aid of the weldingelectrode. A sheet 1 with the coatings 1a, 1b is provided on its upperside with a region 1c which is free of the coating. Opposite to this, onthe lower side, a ringshaped welding region is formed, having a centralcavity 1g, a pointed ring-shaped edge 1g" and a ring-shaped trough lg.For this purpose a double end milling cutter is used, consisting of aninner spherically ended cutter 7c and a ring-shaped endmilling cutter 7dfixed thereto, which leaves standing the ringshaped contact edge 1g"which has a predetermined electric resistance. The prepared sheet 1 isplaced over a sheet 2 to be welded on, the sheet 2 having its uppercoating removed at a region 2c, and the sheet 2 is connected to theelectric-welding circuit. By pressure of an electrode 5, the ring-shapedcontact edge is pressed onto the sheet 2 and the welding is thusperformed. When the milling is effected in separate-working stages, aguide sleeve 6 may be inserted in the outer ringshaped milling 1g so asto be precisely centered, and then the central milling 1g is produced,using the guide sleeve. This procedure is not illustrated in thedrawings. Alternatively, the pressing forward of the ring-shaped contactedge may take place after the welding current has been switched on sothat a delay is introduced which shortens the welding time. As can beseen from FIG. 8, the welding region can alternatively be embossedbefore the actual welding step. The increasing spacings of the surfacesto be welded, as viewed from the tip of the ring-shaped contact edgetowards the cavity, favors the production of electric arcs in the regionof the cavity, and limits the heating at this region. The lowermostcoating 21: of the sheet 2 remains undamaged.

In accordance with a fourth example, the method may also be applied toseam welding. In FIG. 9 the coating has been removed in a strip-shapedregion and the punch 3 and the opening 4a of the die 4' arestrip-shaped. Accordingly, a ridge Id is embossed in the sheet 1 havingthe coatings Ia, lb. Milling is performed using a disc-shaped millingtool on a horizontal shaft 7p. As will be seen from FIG. 9, the millingtool has a groove-milling region 7n at its center for milling arectangular channel and at its two sides it has groove-milling portions7q for milling channels with bevelled sides. Thus, when the milling isperformed, a central channel 11 results, with two-side channels Iq,which are flanked by contact edges Iq" and Iq'. The central groove isdeeper so that arcs occur only in the channels 1q, and two-welded seamsare produced in the sheet, with only slight heating.

According to a fifth example, illustrated in FIGS. and 11, a thick sheetwhich cannot be embossed is welded onto a thinner sheet. The thick sheet1 has coatings 1a, 1b and is provided at its upper side with a milledout region 1v and opposite thereto, at its lower side, with aring-shaped welding region. For forming the ring-shaped welding region,a double end milling cutter is used, consisting of an inner flat endedcutter 7e and an outer annular cutter 7f fixedly connected thereto, thecutter 7]" having a recess 7f at its inner circumference. This recessleaves an edge 1h" standing in the material of the sheet I, which isbordered on the inside by a cavity 1!: and on the outside by aring-shaped recess 1h. The prepared sheet I is placed over a sheet 2having the coatings 2a, 2b with a portion 2c of the upper coatingremoved, and the sheet 2 is connected to a capacitor-weldinginstallation C by a cable 10, and the electrode 5 is connected to thewelding installation by a cable 9. By pressing down the electrode thethin region of the sheet 1 between the recess IV and the cavity 1b ispressed downwardly into contact with the sheet 2, so that welding takesplace. The lowermost coating 2b remains undamaged.

The ring-shaped recess 7f in the milling cutter 7f determines the heightand the cross section of the contact edge 1h" formed by the milling, andthus determines its electric resistance. Instead of using a two-partmilling cutter as shown in FIG. 10, a one-piece milling cutter may beused provided it has a recess corresponding to the recess 7f of FIG. 10.

In accordance with a sixth example shown in FIGS. 12 and 13, theformation of electric arcs at the middle of the welding region ispromoted. A sheet I having the coatings 1a, lb is provided at its lowerside with a ring-shaped welding region. For this, a double end millingcutter is used, consisting of an inner cutter 7h which has a flat endbevelled at its margin, and an outer ring-shaped cutter 71' fixedthereto. The bevelled margin of the inner cutter leaves an inwardlytapering contact edge lj" standing, which is inwardly bordered by acavity lj and outwardly bordered by a ring-shaped milled out region 1;.The prepared sheet I is placed on a sheet 2 having the coatings 2a, 2band having the coating 20 removed at a region 20, and the sheets areconnected to a capacitor-welding installation C by means of cables 9 and10. The welding is per fonned under the pressure of a ram (notillustrated). The inwardly tapering side of the ring-shaped contact edgefavors the production of electric arcs in the cavity.

In a seventh example illustrated in FIGS. 14 and 15, the formation ofelectric arcs at the outside of the cavity is promoted, and the pressingthrough of the weld region is facilitated. A sheet 1 having coatings 1a,lb is provided on its lower side with a ring-shaped welding region. Forthis purpose a double end milling cutter is used, consisting of aninner-milling cutter 7k having a flat end, and an outer ring-shapedmilling cutter 7l fixedly connected therewith, the cutter 71 beingbevelled at its inner and outer end regions. The inner bevel allows aninwardly tapering contact edge lit" to remain standing, which isbordered on the inside by a cavity 1k and on the outside by a milledregion 1k having a trapezium-shaped section. The prepared sheet 1 isplaced with its weld region centrically over the stripped area 20 of thesheet 2, which has the coatings 2a, 2b. The sheet I is connected to acapacitor discharge welding installation C by a cable 9, and the sheet 2is connected to the welding installation C by a cable 10. The welding isperformed under the pressure of the ram 8, which does not conductcurrent. The lowermost coating 2b of the sheet 2 remains undamaged.

In accordance with an eighth example, more particularly for thin sheetswhich cannot be milled out, embossings as shown in FIGS. 16 and 17 areprovided. A sheet 1 coated on both sides has a region 10 at its lowerside where the coating has been removed. As shown in FIG. 16, with theaid of a punch and a die, a downwardly directed protrusion is formed,and on the protrusion a pointed inwardly tapering edge I!" and a cavity11 are embossed. In the form shown in FIG. 17, only a downwardlyprojecting ring-shaped contact edge 1m" is embossed, the cross sectionof which has the shape of a triangle. The sheet 2 to be welded on, andwhich is coated on both sides, has a region 20 on its upper side, fromwhich the coating has been removed. After connecting the sheets 1 and 2to a welding installation C by cables 9 and 10, the welding is performedunder the pressure of a ram (not illustrated). The outer coatings 1a and2b remain undamaged. For seam welding, the contact edges and cavitiesare given a rectilinear shape, instead of a circular shape.

In the ninth example, welding studs or rivetlike members are used. InFIG. 18, an upper sheet I having coatings 1a and 1b is shown, the sheethaving a stepped bore 1r and having a region 10 at its lower side wherethe coating 1b has been removed. A lower sheet 2, having coatings onboth sides but with a region 2c where the coating has been removed fromthe upper side is positioned underneath the sheet 1. In the bore of theupper sheet, a welding rivet 14 is inserted, the rivet having a widenedhead, and a cavity at its lower end, serving for forming a ring-shapedcontact edge 14b. An electrode 5 which engages the head of the rivet 14is connected by a cable 9 to a capacitor discharge welding installationC and the lower sheet 2 is connected by a cable 10 to the weldinginstallation C. The lower end of the rivet 14 is welded to the sheet 2,under the pressure exerted by the electrode. Since the shank of thewelding rivet is not welded to the upper sheet, the upper sheet can berotated about the shank relative to the lower sheet, after the welding.The lower coating of the sheet 2 remains undamaged.

In the modification shown in FIG. 19, the upper sheet I is provided witha bore It in which a welding rivet 15 is inserted which has a head atits upper end, and at its lower end has a welding stud 15a and aring-shaped contact edge 15b, these facing a region of a double sidedcoated lower sheet 22, where the coating has been removed. Asound-damping layer 21 is provided on the lower coating 22b of the sheet22, and a further sheet 2 having coatings 2a and 2b on its sides isadhesively bonded to layer 21. An electrode 5 is connected to a weldinginstallation C by means of a cable 9, and the sheet 22 is connected tothe welding installation by a cable 10. Under the pressure of theelectrode, electric arcs are struck, and the welding takes place betweenthe lower end of the welding rivet and the sheet 22. Since the shank ofthe rivet is not welded to the sheet 1, the sheets can be rotatedrelative to each other.

In the 10th example, high-transition resistances between sheets areobtained by the use of welding balls. In FIG. 20, an upper sheet 1 isshown having coatings 1a and 1b, with the coating lb removed at a region1c, and a lower sheet 2 having coatings 2a, 2b with the coating 2aremoved at a region 20. In the stripped regions, ring-shaped grooves Isof triangular cross section are milled into the sheets. Balls 16 areinserted in the grooves, their diameters being so large that the sheetsare held apart at a spacing suitable for enabling the welding to beperformed. The welding is then carried out using a ram 8 which does notcarry electric current. The small areas of contact established by theballs with the sheets, enable the necessary high-electric resistance tobe obtained. The outer coatings 1a, 2b remain undamaged.

In FIG. 21, an upper sheet I having the coatings 1a and 1b, with theregions 1c, Ic' thereof removed, and with bores 11 provided within thestripped regions is shown, the bores serving for receiving welding balls17. The welding balls have a diameter which corresponds to approximatelytwice the sheet thickness and which is slightly smaller than thediameter of the bores. A lower sheet 2 having coatings 2a, 2b has aregion 20 on its upper side where the coating has been removed. Thelower sheet 2 is connected to an electric-welding installation, and anelectrode 5 is also connected to the welding installa tion. The weldingis performed under the pressure of the electrode, so that the balls arewelded to both sheets. The lower coatings remains undamaged.

In an llth example, illustrated in FIGS. 22 and 23, an upper sheet 1having the coatings la and lb is provided at its lower side with apointed welding region by milling out. For this purpose, an end-millingcutter 7m is used, which has a shallow conical recess at its middle. Itproduces a flat ring-shaped depression lp in which there is a shallowconical region with the tip 1p. Coaxial with this milling, the coatingis removed from a region 10' at the lower side of the sheet 1, andopposite thereto on the sheet 2 which has the coatings 2a, 2b, a region2c is removed from the coating 2a. An intermediate welding piece 11 isinserted between the sheets, its lower side having a cavity and aring-shaped contact edge 11b. The sheet 1 is connected to acapacitor-welding installation C by a cable 9, and the sheet 2 by acable 10. The welding is performed using the pressure of a ram (notillustrated). By means of the tip 1p on the sheet 1 and of thering-shaped contact edge llb of the intermediate welding piece, electricarcs are struck, the surfaces are caused to melt and are welded togetherunder pressure. The lower coatings remain undamaged.

In the 12th example, the high-transition resistance necessary for shorttime welding, is achieved by using small insert pieces which are pointedand which have fluted or knurled margins. In FIG. 24, an upper sheet 1is shown having the coatings la and 1b and which at the welding regionhas its upper coating removed at a region 1c and its lower coatingremoved at a region 1c. The lower sheet 2 having the coatings 2a and 2bhas a stripped region only at its upper side. Between the bare surfaces,an intermediate welding piece 18 is inserted which is hollow and whichis in the form of a pointed cone, so that it contacts the upper sheetonly at its apex and contacts the lower sheet with a ring-shaped edge18b. After connecting the lower sheet to a welding installation, thewelding is performed by pressing down the upper sheet with the aid of anelectrode (not illustrated).

In the arrangement shown in FIG. 25, an upper sheet I having coatings laand lb is provided at its lower side with a region 1c where the coatingis removed, the sheet being arranged over a lower sheet 2 havingcoatings 2a and 2b, with a region 2c in the upper side thereof where thecoating has been removed. An intermediate welding piece in the form of aspherical segment 12 is inserted between the bare regions of the sheets,the segment 12 having at its lower side a ringshaped edge 12b, andmaking point contact at its top. The sheets 1 and 2 are connected to acapacitor-welding installation C by cables 9 and 10. The welding isperformed under the pressure of a ram (not illustrated) and the externalcoatings remain undamaged.

In the modification shown in FIG. 26, the sheets 1 and 2 are providedwith coating removed regions only at the sides thereof which face eachother, and an intermediate welding piece Ila is inserted between them,which both at its upper and lower ends has a cavity and a pointedring-shaped contact edge 11b. The welding is carried out as in theexample illustrated in FIG. 25, the construction being suitable forresistance welding and also for electric arc welding.

In the 13th example, illustrated in FIGS. 27 to 29, sheets are welded toeach other which are coated only on one side, more particularly aluminumsheets or superimposed sheets which are provided on their opposite sideswith a metallic electrically conductive coating. In FIG. 27, a sheet Iis shown which has a coating la only at its upper side, a region 1chaving been removed from the coating 1a, and a lower sheet 2 which isprovided with a coating 2b only on its lower side. Between the sheets,an insulating insert 19 is placed or bonded which has holes arranged tocorrespond to the welding regions. In these holes, intermediate weldingpieces in the form of sharpedged rings 13 of thin sheet metal areinserted, their height being about double the thickness of theinsulating insert. After connecting the lower sheet to a weldinginstallation, the welding is carried out by pressing down an electrode,(not illustrated), onto the sheet 1. The sharp margin of the rings 13produces the desired high-electric resistance. The lowermost coating 2bremains undamaged.

Instead of using a ring, a plate 13a as shown in FIG. 28 may be used,the plate having spikes at its margin, which are alternately bentupwardly and downwardly. The points of the spikes result in an electricpoint contact being established.

When the cavity between the sheets 1 and 2 is larger and is to be filledwith material, then a ring 13b as shown in FIG. 29 is used, the crosssection of which is a rombus or square so disposed that its comers fonnthe upper and lower edges of the ring.

The profiles illustrated in cross section in FIGS. 3, 5, 7, ll, 13, 15and 23 may be formed in a shape suitable for seam welding, by means ofdisc-milling tools such as shown in FIG. 9, so that the profile crosssections have rectilinear contact edges formed by grooves and ridges,the milling tools being formed by suitable combinations of millingcutters. Instead of guiding the milling cutter by the rectilinearopening of the die 4, other rectilinear guide elements may be used whichengage with the groove downwardly embossed in the sheet I so as to givean accurate rectilinear guidance to the milling cutter, in relation tothe groove. When no embossed regions are present, millings in the lowerwelding surface of the sheet I may serve for arranging the guidingelements for cylindrical or for rectilinear milling.

For limiting the milling depth, the arrangement shown in FIG. 10provides a ring-shaped recess 7f by which the height of the ring-shapedcontact edge lh" (FIG. 11) is determined. The same is true for themilling cutters of FIGS. 6, l2 and 14 for the milling cutter of FIG. 9.

Instead of determining the milling depth in this manner, abutmentelements, more particularly adjustable elements, may be provided on theend-milling cutters, which selectively and precisely limit the movementperpendicular to the sheet 1. Corresponding elements may also bearranged on the shaft 7p. This arrangement can also be made use of inrespect of the guide elements 6, dies 4, 4 or other parts associatedtherewith.

Iclaim:

I. In a method of welding metal sheets coated with a substantiallynonconductive coating to each other by means of the electric current,said method comprising a. removing the coating at least from the inneropposed faces of said coated metal sheets to provide said sheets withblank opposed welding areas,

b. providing at least one of said blank welding areas with knifelikemetallic edges,

c. establishing contact of said knifelike edges with the blank weldingareas,

d. supplying the electric-welding current indirectly through at leastone of the metal layers of said sheets, thereby establishing electricalresistance, and

e. electrically welding the two metal sheets together at the weldingareas, whereby f. at least one of the outer coatings of said metalsheets is kept intact.

2. The method of claim 1, wherein in step (b) the knifelike metallicedges are annular edges.

3. The method of claim 1, wherein in step (b) knifelike metallic edgesare elongated edges.

4. The method of claim 1, wherein in step (b) the knifelike edges areedges produced by embossing.

5. The method of claim 1, wherein in step (b) the knifelike edges areedges produced by milling.

6. The method of claim 1, wherein in step (b) the knifelike metallicedges are edges produced by first cylindrically embossing one of saidsheets and then milling the embossed projected part of said sheet so asto form an annular knife-shaped edge and a cavity within said annularedge.

7. The method of claim 1, wherein in step (b) a central cavity and atthe periphery thereof an annular milled recess are produced in one ofthe metal sheets to form a knifelike metallic edge between the cavityand the peripheral annular recess and wherein in step (c) the knifelikemetallic edge of said one metal sheet between the central cavity and theperipheral annular recess is exposed to the downward pressure of a punchto cause said knifelike metal edge to contact the opposed blank weldingarea of the other metal sheet.

8. The method of claim 1, wherein in step (b) a central cavity and atthe periphery thereof an annular milled recess are produced in one ofthe metal sheets to form a knifelike metallic edge between the cavityand the peripheral annular recess and wherein in step (c) the knifelikemetallic edge of said one metal sheet between the central cavity and theperipheral annular recess is exposed to the downward pressure of anelectrode to cause said knifelike metal edge to contact the opposedblank welding area of the other metal sheet.

9. The method of claim 1, wherein for seam welding in step (a) thecoating is removed in a continuous seam from at least the inner opposedfaces of the coated metal sheets to provide said sheets with continuousblank opposed welding seams, while in step (b) the knifelike metallicedges are produced by first continuously embossing one of said sheets toform a continuous projection of said sheet, continuously milling out ofsaid projection a central groove and on both sides of said centralgroove groove-shaped recesses, thereby preserving knifelike metallicedges between said central groove and said groove-shaped recesses,whereafter in step (c) contact is established between the continuousknifelike metallic edges and the opposed blank welding seam of the othermetal sheet to be welded.

10. The method of claim I, wherein in step (a) the outer surface of oneof the coated metal sheets is additionally provided with a shallowmilled recess opposite to the welding area provided in step (b) with theknifelike metallic edges.

ll. The method of claim 1, wherein in step (b) a central cylindricalcavity and at the periphery thereof an annular recess are produced toform an annular knifelike metallic edge between the central cylindricalcavity and the peripheral annular recess by milling with a millingcutter having an outermilling cutter for cutting said annular recess, acentral-milling cutter having a flat-cutting end face, and a recessbetween said outer-milling cutter and said central-milling cutter.

12. The method of claim 1, wherein in step (b) a central cavity and atthe periphery thereof an annular recess are produced with a millingcutter having an outer-milling cutter with a plane-cutting end face anda central-milling cutter bevelled at its circumference to form aninwardly bevelled annular knifelike metallic edge between the centralcavity and the peripheral annular recess.

13. The method of claim 1, wherein in step (b) a central cylindricalcavity and at the periphery thereof an annular recess of trapezoidalprofile are produced by milling with a milling cutter having anouter-milling cutter bevelled at its outer and inner periphery and acentral-milling cutter having a flat-cutting end face to form anoutwardly bevelled annular knifelike metallic edge between the centralcavity and the peripheral annular recess.

14. The method of claim 1, wherein in step (b) the knifelike metallicedges are edges produced by embossing one of said metal sheets to form acircular depression with an annular knifelike edge encircling a cavity.

15. The method of claim 1, wherein in step (a) the one coated metalsheet is provided with a differential diameter bore and wherein in step(b) a welding rivet is inserted into said bore, the end face of theshank of said rivet having an annular knifelike edge surrounding acentral recess.

16. The method of claim 1, wherein in step (a) the one coated metalsheet is provided with a bore and wherein in step (b) a welding rivet isinserted into said bore, the end of the shank of said rivet beingprovided with an annular knifelike edge and with a central-welding pin.

17. The method of claim 1, wherein in step (b) grooves of triangularprofile are milled out on the blank opposed welding areas of the metalsheets to be welded and wherein welding balls are inserted into saidjuxtapositioned grooves to provide the knifelike metallic edges.

18. The method of claim 1, wherein in step (a) the one coated metalsheet is provided with bores and wherein in step (b) welding balls areinserted into said bores, said welding balls protruding on top andbottom from said bores and providing the knifelike metallic edges.

19. The method of claim I, wherein in step (b) a welding point isprovided by milling at the blank welding area of one of the metal sheetsand an intermediate welding piece having an annular knifelike edge isinserted between the point of said one metal sheet and the blank opposedcircular welding area of the other metal sheet, whereafter in step (c)contact of said welding point with one surface of the intermediatewelding piece and of the blank circular welding area of the other metalsheet with the annular knifelike edge of the intermediate welding pieceis established.

20. The method according to claim 1, wherein in step (b) a hollowintermediate welding piece is inserted between the blank opposed weldingareas of the metal sheets, said intermediate welding piece beingprovided with a contact point on its top face and with an annularknifelike edge on its bottom face.

21. The method of claim 1, wherein in step (b) an intermediate weldingpiece provided with annular knifelike edges on its top and bottom facesis inserted between the blank opposed welding areas of the metal sheets.

22. The method of claim 1, wherein in step (b) an insulating layer isplaced between the metal sheets having a coating on their outer faces,said insulating layer having holes at the welding areas and wherein theknifelike metallic edges are provided by inserting into the holesinserts with such knifelike metallic edges.

23. The method of claim 22, wherein the insert is a sharpedged ring ofsheet metal.

24. The method of claim 22, wherein the insert is a wire having arhomboidal cross section and being inserted into the holes of theinsulating layer so as to stand on a corner.

25. The method of claim 22, wherein the insert is a disc havingalternately upwardly and downwardly bent prongs.

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2. The method of claim 1, wherein in step (b) the knifelike metallicedges are annular edges.
 3. The method of claim 1, wherein in step (b)knifelike metallic edges are elongated edges.
 4. The method of claim 1,wherein in step (b) the knifelike edges are edges produced by embossing.5. The method of claim 1, wherein in step (b) the knifelike edges areedges produced by milling.
 6. The method of claim 1, wherein in step (b)the knifelike metallic edges are edges produced by first cylindricallyembossing one of said sheets and then milling the embossed projectedpart of said sheet so as to form an annular knife-shaped edge and acavity within said annular edge.
 7. The method of claim 1, wherein instep (b) a central cavity and at the periphery thereof an annular milledrecess are produced in one of the metal sheets to form a knifelikemetallic edge between the cavity and the peripheral annular recess andwherein in step (c) the knifelike metallic edge of said one metal sheetbetween the central cavity and the peripheral annular recess is exposedto the downward pressure of a punch to cause said knifelike metal edgeto contact the opposed blank welding area of the other metal sheet. 8.The method of claim 1, wherein in step (b) a central cavity and at theperiphery thereof an annular milled recess are produced in one of themetal sheets to form a knifelike metallic edge between the cavity andthe peripheral annular recess and wherein in step (c) the knifelikemetallic edge of said one metal sheet between the central cavity and theperipheral annular recess is exposed to the downward pressure of anelectrode to cause said knifelike metal edge to contact the opposedblank welding area of the other metal sheet.
 9. The method of claim 1,wherein for seam welding in step (a) the coating is removed in acontinuous seam from at least the inner opposed faces of the coatedmetal sheets to provide said sheets with continuous blank opposedwelding seams, while in step (b) the knifelike metallic edges areproduced by first continuously embossing one of said sheets to form acontinuous projection of said sheet, continuously milling out of saidprojection a central groove and on both sides of said central groovegroove-shaped recesses, thereby preserving knifelike metallic edgesbetween said central groove and said groove-shaped recesses, whereafterin step (c) contact is established between the continuous knifelikemetallic edges and the opposed blank welding seam of the other metalsheet to be welded.
 10. The method of claim 1, wherein in step (a) theouter surface of one of the coated metal sheets is additionally providedwith a shallow milled recess opposite to the welding area provided instep (b) with the knifelike metallic edges.
 11. The method of claim 1,wherein in step (b) a central cylindrical cavity and at the peripherythereof an annular recess are produced to form an annular knifelikemetallic edge between the central cylindrical cavity and the peripheralannular recess by milling with a milling cutter having an outer-millingcutter for cutting said annular recess, a central-milling cutter havinga flat-cutting end face, and a recess between said outer-milling cutterand said central-milling cutter.
 12. The method of claim 1, wherein instep (b) a central cavity and at the periphery thereof an annular recessare produced with a milling cutter having an outer-milling cutter with aplane-cutting end face and a central-milling cutter bevelled at itscircumference to form an inwardly bevelled annular knifelike metallicedge between the central cavity and the peripheral annular recess. 13.The method of claim 1, wherein in step (b) a central cylindrical cavityand at the periphery thereof an annular recess of trapezoidal profileare produced by milling with a milling cutter having an outer-millingcutter bevelled at its outer and inner periphery and a central-millingcutter having a flat-cutting end face to form an outwardly bevelledannular knifelike metallic edge between the central cavity and theperipheral annular recess.
 14. The method of claim 1, wherein in step(b) the knifelike metallic edges are edges produced by embossing one ofsaid metal sheets to form a circular depression with an annularknifelike edge encircling a cavity.
 15. The method of claim 1, whereinin step (a) the one coated metal sheet is provided with a differentialdiameter bore and wherein in step (b) a welding rivet is inserted intosaid bore, the end face of the shank of said rivet having an annularknifelike edge surrounding a central recess.
 16. The method of claim 1,wherein in step (a) the one coated metal sheet is provided with a boreand wherein in step (b) a welding rivet is inserted into said bore, theend of the shank of said rivet being provided with an annular knifelikeedge and with a central-welding pin.
 17. The method of claim 1, whereinin step (b) grooves of triangular profile are milled out on the blankopposed welding areas of the metal sheets to be welded and whereinwelding balls are inserted into said juxtapositioned grooves to providethe knifelike metallic edges.
 18. The method of claim 1, wherein in step(a) the one coated metal sheet is provided with bores and wherein instep (b) welding balls are inserted into said bores, said welding ballsprotruding on top and bottom from said bores and providing the knifelikemetallic edges.
 19. The method of claim 1, wherein in step (b) a weldingpoint is provided by milling at the blank welding area of one of themetal sheets and an intermediate welding piece having an annularknifelike edge is inserted between the point of said one metal sheet andthe blank opposed circular welding area of the other metal sheet,whereafter in step (c) contact of said welding point with one surface ofthe intermediate welding piece and of the blank circular welding area ofthe other metal sheet with the annular knifelike edge of theintermediate welding piece is established.
 20. The method according toclaim 1, wherein in step (b) a hollow intermediate welding piece isinserted between the blank opposed welding areas of the metal sheets,said intermediate welding piece being provided with a contact point onits top face and with an annular knifelike edge on its bottom face. 21.The method of claim 1, wherein in step (b) an intermediate welding pieceprovided with annular knifelike edges on its top and bottom faces isinserted between the blank opposed welding areas of the metal sheets.22. The method of claim 1, wherein in step (b) an insulating layer isplaced between the metal sheets having a coating on their outer faces,said insulating layer having holes at the welding areas and wherein theknifelike metallic edges are provided by inserting into the holesinserts with such knifelike metallic edges.
 23. The method of claim 22,wherein the insert is a sharp-edged ring of sheet metal.
 24. The methodof claim 22, wherein the insert is a wire having a rhomboidal crosssection and being inserted into the holes of the insulating layer so asto stand on a corner.
 25. The method of claim 22, wherein the insert isa disc having alternately upwardly and downwardly bent prongs.